Silver linings - new research reveals the science behind the Saxifrage's silver-white crust

Cambridge University plant scientists and horticultural staff combine forces to discover the how and why behind the Saxifraga’s silver-white crust.

Paul Aston, Raymond Wightman, Simon Wallis in the Garden's Alpine yard with some of the National Collection of Saxifraga

Staff from Cambridge University Botanic Garden and the Microscopy Facility, based at the Sainsbury Laboratory, Cambridge University (SLCU), reveal the inner workings of Saxifraga plants which explain the silvery-white crust found on many of species Saxifraga - for the first time in 150 years.

The Garden’s National Collection of Saxifraga has recently been the focus of newly published research explaining the leaf structures responsible for the silvery-white crust that forms on some alpine Saxifrages (Saxifraga). Published in the journal Flora, the study looked at the different types of cells that help to make the crust, how these structures form and what exactly they deposit onto the leaf to form the silvery-white crust which many Saxifraga are so admired for.

Paul Aston, Alpine and Woodland Supervisor at the Botanic Garden and responsible with his Assistant, Simon Wallis, for growing and maintaining the Garden’s National Collection of Saxifraga says: “It’s been incredibly exciting to work on this project and see part of the Garden’s collection that I’m responsible for being used for such pioneering research. This is exactly what having a University Botanic Garden is all about.”

Dr Raymond Wightman, Microscopy Core Facility Manager at SLCU says: “These latest discoveries into how Saxifraga produce and lay down their silvery-white crust is very exciting and has been made possible thanks to new microscope technologies along with the availability of the wild plant material, held and looked after at the Botanic Garden. Because of this we now have a completely new insight into the biology of this wonderful genus which may be able to help us develop new biomaterials for pharmaceuticals and engineering applications.”

Before this study, the only insight into Saxifraga leaf anatomy, development and chemistry came from 19th century German works.

Raymond explains: “Our understanding of what leaf secretory structures, called “hydathodes”, look like and how they are formed comes from very distantly related plants and, it turns out, Saxifraga have important differences.”

There are approximately 110 species and hybrids of Saxifraga originating from Europe and Asia in the Garden’s National Collection. The study focused on one plant Saxifraga cochlearis, which provided an interesting source of material to Raymond because of the way the hydathodes are arranged in a regular fashion along the length of the leaf. On a single leaf you can observe the hydathodes becoming more developed as you move progressively to the leaf tip, starting off as a group of small cells and ending as a volcano-type structure that spews out of the crust.

“I needed test subjects for new, state-of-the art microscopes which allowed me to look inside plant parts, cells and even parts of cells and to do chemical imaging”, explains Raymond. “We knew everyday laboratory plants - such as Arabidopsis thaliana, the plant science world’s equivalent of the fruit fly - worked fine but did not know how useful the equipment would be beyond using these. This is where working in the grounds of the Botanic Garden and having the Garden’s wonderful collection of plants is so crucial to plant science research. Paul and Simon introduced me to the encrusted Saxifraga . Their tough, leathery and crusty leaves provided a challenge for the process of cryo-scanning electron microscopy, a type of microscopy that relies on low temperatures of minus 150 degrees Celsius, but the equipment worked remarkably well and we were fortunate to come up with amazing results.

“While opening up and looking inside the leaves, during a process called cryo-fracture, we realised that we were seeing things that we did not recognise from the text books. The chemical imaging also revealed other things (such as the precise arrangement of molecules in the crust) that had not been discovered before. We were able to take hundreds of pictures to back up our findings. Paul and Simon were always on hand to direct me to the best plants to study and to help figure out what we were looking at. My entire background of Saxifraga ecology came from these two people – it was a perfect combination.”

The results of the study will not only satisfy the curiosity of plant lovers but also demonstrate how plant science is at a pioneering stage. Raymond explains: “Modern microscopy techniques are now no longer confining plant scientists to a small number of ‘model’ plants grown in the laboratory environment. This really shows how we can leap out in to the unknown and observe things that were not possible for plant scientists just two years ago. This is just the very first part of our work on Saxifraga biology. It is all very exciting so watch this space!”

Paul Aston, concludes: “Raymond was genuinely curious about the plant material we had behind the scenes. We directed him to the Alpine collection of plants as their small stature made them ideal candidates for the microscopy equipment and the broad range of material from this genus meant Raymond had plenty of material to work with. Having a diverse range of wild collected material has really helped to pull out some interesting variations in the genus, not only in hydathode positioning on the leaf surface but also in the composition of the encrustations they exude. Our experience of growing them and knowledge about the sections of Saxifraga we grow has helped to point out some of these patterns. Raymond has been great fun to work with and it’s very rewarding for us to see the results of this partnership of the Garden and Laboratory working together.”

Professor Beverley Glover, Director of Cambridge University Botanic Garden says: “Our living collection of 8000 plant species is often the focus of new scientific research, but it’s particularly pleasing in this study to see the expertise of the horticultural staff who grow the plants included in the research. I’m looking forward to many more such studies as the Garden’s reputation as an international centre of excellence in plant diversity grows”.

To read more about the Garden’s National Collection of Saxifraga click on the link to the left